30个python常用技巧

目录
1.原地调换数字

 

python提供了一个直观的方法,去用一行代码解决变量交换问题.请看如下示例:

x, y = 10, 20
print(x, y)
 
x, y = y, x
print(x, y)
 


#1 (10, 20)
#2 (20, 10)



The assignment on the right seeds a new tuple. While the left one instantly unpacks that (unreferenced) tuple to the names <a> and <b>.Once the assignment is through, the new tuple gets unreferenced and flagged for garbage collection. The swapping of variables also occurs at eventually.
右边的作业是一个新的元组,左边的作业立刻拆解得到a,b的值,新任务完成,右边的新的元组随之进入系统回收.这样值的交换就实现了

2.比较运算符的连接

Aggregation of comparison operators is another trick that can come handy at times.

n = 10
result = 1 < n < 20
print(result)

# True

result = 1 > n <= 9
print(result)

# False

3.使用三元运算给条件赋值

Ternary operators are a shortcut for an if-else statement and also known as conditional operators.

[on_true] if [expression] else [on_false]
Here are a few examples which you can use to make your code compact and concise.

The below statement is doing the same what it is meant to i.e. “assign 10 to x if y is 9, otherwise assign 20 to x“. We can though extend the chaining of operators if required.

x = 10 if (y == 9) else 20
Likewise, we can do the same for class objects.

x = (classA if y == 1 else classB)(param1, param2)
In the above example, classA and classB are two classes and one of the class constructors would get called.


 
Below is one more example with a no. of conditions joining to evaluate the smallest number.

def small(a, b, c):
    return a if a <= b and a <= c else (b if b <= a and b <= c else c)
    
print(small(1, 0, 1))
print(small(1, 2, 2))
print(small(2, 2, 3))
print(small(5, 4, 3))

#Output
#0 #1 #2 #3
We can even use a ternary operator with the list comprehension.

[m**2 if m > 10 else m**4 for m in range(50)]

#=> [0, 1, 16, 81, 256, 625, 1296, 2401, 4096, 6561, 10000, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400, 441, 484, 529, 576, 625, 676, 729, 784, 841, 900, 961, 1024, 1089, 1156, 1225, 1296, 1369, 1444, 1521, 1600, 1681, 1764, 1849, 1936, 2025, 2116, 2209, 2304, 2401]

4.使用多行字符串

The basic approach is to use backslashes which derive itself from C language.

multiStr = "select * from multi_row \
where row_id < 5"
print(multiStr)

# select * from multi_row where row_id < 5
One more trick is to use the triple-quotes.

multiStr = """select * from multi_row 
where row_id < 5"""
print(multiStr)

#select * from multi_row 
#where row_id < 5
The common issue with the above methods is the lack of proper indentation. If we try to indent, it’ll insert whitespaces in the string.

So the final solution is to split the string into multi lines and enclose the entire string in parenthesis.

multiStr= ("select * from multi_row "
"where row_id < 5 "
"order by age") 
print(multiStr)

#select * from multi_row where row_id < 5 order by age

5.将列表元素储存到新的变量中

We can use a list to initialize a no. of variables. While unpacking the list, the count of variables shouldn’t exceed the no. of elements in the list.

testList = [1,2,3]
x, y, z = testList

print(x, y, z)

#-> 1 2 3

6.用模块打印文件地址

If you want to know the absolute location of modules imported in your code, then use the below trick.

import threading 
import socket

print(threading)
print(socket)

#1- <module 'threading' from '/usr/lib/python2.7/threading.py'>
#2- <module 'socket' from '/usr/lib/python2.7/socket.py'>

7.使用“_”进行交互.

It’s a useful feature which not many of us are aware.

In the Python console, whenever we test an expression or call a function, the result dispatches to a temporary name, _ (an underscore).

>>> 2 + 1
3
>>> _
3
>>> print _
3
The “_” references to the output of the last executed expression.

8.字典或集合的一些领悟

Like we use list comprehensions, we can also use dictionary/set comprehensions. They are simple to use and just as effective. Here is an example.

testDict = {i: i * i for i in xrange(10)} 
testSet = {i * 2 for i in xrange(10)}

print(testSet)
print(testDict)

#set([0, 2, 4, 6, 8, 10, 12, 14, 16, 18])
#{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
Note- There is only a difference of <:> in the two statements. Also, to run the above code in Python3, replace <xrange> with <range>.

9.调试脚本

We can set breakpoints in our Python script with the help of the <pdb> module. Please follow the below example.

import pdb
pdb.set_trace()
We can specify <pdb.set_trace()> anywhere in the script and set a breakpoint there. It’s extremely convenient.

10.设置文件共享

Python allows running an HTTP server which you can use to share files from the server root directory. Below are the commands to start the server.

# Python 2

python -m SimpleHTTPServer
# Python 3

python3 -m http.server
Above commands would start a server on the default port i.e. 8000. You can also use a custom port by passing it as the last argument to the above commands.

11.在python中检查对象

We can inspect objects in Python by calling the dir() method. Here is a simple example.

test = [1, 3, 5, 7]
print( dir(test) )
['__add__', '__class__', '__contains__', '__delattr__', '__delitem__', '__delslice__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getslice__', '__gt__', '__hash__', '__iadd__', '__imul__', '__init__', '__iter__', '__le__', '__len__', '__lt__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__', '__rmul__', '__setattr__', '__setitem__', '__setslice__', '__sizeof__', '__str__', '__subclasshook__', 'append', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse', 'sort']

12.简化if语句

To verify multiple values, we can do in the following manner.

if m in [1,3,5,7]:
instead of:

if m==1 or m==3 or m==5 or m==7:
Alternatively, we can use ‘{1,3,5,7}’ instead of ‘[1,3,5,7]’ for ‘in’ operator because ‘set’ can access each element by O(1).

13.python版本检测

Sometimes we may not want to execute our program if the Python engine currently running is less than the supported version. To achieve this, you can use the below coding snippet. It also prints the currently used Python version in a readable format.

import sys

#Detect the Python version currently in use.
if not hasattr(sys, "hexversion") or sys.hexversion != 50660080:
    print("Sorry, you aren't running on Python 3.5\n")
    print("Please upgrade to 3.5.\n")
    sys.exit(1)
    
#Print Python version in a readable format.
print("Current Python version: ", sys.version)
Alternatively, you can usesys.version_info >= (3, 5) to replacesys.hexversion!= 50660080 in the above code. It was a suggestion from one of the informed reader.

Output when running on Python 2.7.

Python 2.7.10 (default, Jul 14 2015, 19:46:27)
[GCC 4.8.2] on linux
   
Sorry, you aren't running on Python 3.5

Please upgrade to 3.5.
Output when running on Python 3.5.

Python 3.5.1 (default, Dec 2015, 13:05:11)
[GCC 4.8.2] on linux
   
Current Python version:  3.5.2 (default, Aug 22 2016, 21:11:05) 
[GCC 5.3.0]

14.拼接字符串

If you want to concatenate all the tokens available in a list, then see the below example.

>>> test = ['I', 'Like', 'Python', 'automation']
Now, let’s create a single string from the elements in the list given above.

>>> print ''.join(test)

15.四个方法反转string/list.

# Reverse The List Itself.

testList = [1, 3, 5]
testList.reverse()
print(testList)

#-> [5, 3, 1]
# Reverse While Iterating In A Loop.

for element in reversed([1,3,5]): print(element)

#1-> 5
#2-> 3
#3-> 1



# Reverse A String In Line.

"Test Python"[::-1]
This gives the output as ”nohtyP tseT”



# Reverse A List Using Slicing.

[1, 3, 5][::-1]
The above command will give the output as [5, 3, 1].

16.玩枚举

With enumerators, it’s easy to find an index while you’re inside a loop.

testlist = [10, 20, 30]
for i, value in enumerate(testlist):
    print(i, ': ', value)

#1-> 0 : 10
#2-> 1 : 20
#3-> 2 : 30

17.在python中使用枚举

With enumerators, it’s easy to find an index while you’re inside a loop.

testlist = [10, 20, 30]
for i, value in enumerate(testlist):
    print(i, ': ', value)

#1-> 0 : 10
#2-> 1 : 20
#3-> 2 : 30

18.从函数中返回多个值

Not many programming languages support this feature. However, functions in Python do return multiple values.

Please refer the below example to see it working.

# function returning multiple values.
def x():
    return 1, 2, 3, 4

# Calling the above function.
a, b, c, d = x()

print(a, b, c, d)

#-> 1 2 3 4

19.Unpack function arguments using splat operator.

The splat operator offers an artistic way to unpack arguments lists. Please refer the below example for clarity.

def test(x, y, z):
    print(x, y, z)

testDict = {'x': 1, 'y': 2, 'z': 3} 
testList = [10, 20, 30]

test(*testDict)
test(**testDict)
test(*testList)

#1-> x y z
#2-> 1 2 3
#3-> 10 20 30

20.使用字典储存函数

We can make a dictionary store expressions.

stdcalc = {
    'sum': lambda x, y: x + y,
    'subtract': lambda x, y: x - y
}

print(stdcalc['sum'](9,3))
print(stdcalc['subtract'](9,3))

#1-> 12
#2-> 6

21.一行代码实现阶乘

Python 2.X.

result = (lambda k: reduce(int.__mul__, range(1,k+1),1))(3)
print(result)
#-> 6
Python 3.X.

import functools
result = (lambda k: functools.reduce(int.__mul__, range(1,k+1),1))(3)
print(result)

#-> 6

22.找出列表中出现最多的值

test = [1,2,3,4,2,2,3,1,4,4,4]
print(max(set(test), key=test.count))

#-> 4

23.重置递归限制

Python restricts recursion limit to 1000. We can though reset its value.

import sys

x=1001
print(sys.getrecursionlimit())

sys.setrecursionlimit(x)
print(sys.getrecursionlimit())

#1-> 1000
#2-> 1001
Please apply the above trick only if you need it.

24.检查对象的内存使用情况

In Python 2.7, a 32-bit integer consumes 24-bytes whereas it utilizes 28-bytes in Python 3.5. To verify the memory usage, we can call the <getsizeof> method.

In Python 2.7.

import sys
x=1
print(sys.getsizeof(x))

#-> 24
In Python 3.5.

import sys
x=1
print(sys.getsizeof(x))

#-> 28

25.使用 __slots__ 节省内存.

Have you ever observed your Python application consuming a lot of resources especially memory? Here is one trick which uses <__slots__> class variable to reduce memory overhead to some extent.

import sys
class FileSystem(object):

    def __init__(self, files, folders, devices):
        self.files = files
        self.folders = folders
        self.devices = devices

print(sys.getsizeof( FileSystem ))

class FileSystem1(object):

    __slots__ = ['files', 'folders', 'devices']
    
    def __init__(self, files, folders, devices):
        self.files = files
        self.folders = folders
        self.devices = devices

print(sys.getsizeof( FileSystem1 ))

#In Python 3.5
#1-> 1016
#2-> 888
Clearly, you can see from the results that there are savings in memory usage. But you should use __slots__ when the memory overhead of a class is unnecessarily large. Do it only after profiling the application. Otherwise, you’ll make the code difficult to change and with no real benefit.

26.Lambda to imitate print function.

import sys
lprint=lambda *args:sys.stdout.write(" ".join(map(str,args)))
lprint("python", "tips",1000,1001)

#-> python tips 1000 1001

27.Create a dictionary from two related sequences.

t1 = (1, 2, 3)
t2 = (10, 20, 30)

print(dict (zip(t1,t2)))

#-> {1: 10, 2: 20, 3: 30}

28.在字符串中寻找多个前缀

print("http://www.google.com".startswith(("http://", "https://")))
print("http://www.google.co.uk".endswith((".com", ".co.uk")))

#1-> True
#2-> True

29.不用循环创造一个列表

import itertools
test = [[-1, -2], [30, 40], [25, 35]]
print(list(itertools.chain.from_iterable(test)))

#-> [-1, -2, 30, 40, 25, 35]
If you have an input list with nested lists or tuples as elements, then use the below trick. However, the limitation here is that it’s using a for Loop.

def unifylist(l_input, l_target):
    for it in l_input:
        if isinstance(it, list):
            unifylist(it, l_target)
        elif isinstance(it, tuple):
            unifylist(list(it), l_target)
        else:
            l_target.append(it)
    return l_target

test =  [[-1, -2], [1,2,3, [4,(5,[6,7])]], (30, 40), [25, 35]]

print(unifylist(test,[]))

#Output => [-1, -2, 1, 2, 3, 4, 5, 6, 7, 30, 40, 25, 35]
Another simpler method to unify the list containing lists and tuples is by using the Python’s <more_itertools> package. It doesn’t require looping. Just do a <pip install more_itertools>, if not already have it.

import more_itertools

test = [[-1, -2], [1, 2, 3, [4, (5, [6, 7])]], (30, 40), [25, 35]]

print(list(more_itertools.collapse(test)))

#Output=> [-1, -2, 1, 2, 3, 4, 5, 6, 7, 30, 40, 25, 35]

30.在python中实现一个真正的开关盒

 

Here is the code that uses a dictionary to imitate a switch-case construct.

def xswitch(x): 
    return xswitch._system_dict.get(x, None) 

xswitch._system_dict = {'files': 10, 'folders': 5, 'devices': 2}

print(xswitch('default'))
print(xswitch('devices'))

#1-> None
#2-> 2